It is unclear why Paul Peters targeted 18-year-old Madeleine Pulver. U.S. federal court documents show Peters once worked for a company with links to her family, but the Pulvers have repeatedly said they don't know him.

By NBC News and msnbc.com news services

SYDNEY, Australia - An Australian investment banker pleaded guilty Thursday to chaining a fake bomb to a young woman's neck in a bizarre extortion bid, before fleeing to the United States.

Paul Peters' lawyer Kathy Crittenden pleaded guilty on his behalf in a Sydney courtroom to a charge of aggravated break and enter and committing a serious indictable offense by knowingly detaining Madeleine Pulver, 18.

Pulver was alone studying in her family's Sydney mansion on Aug. 3 when the 50-year-old Peters, wearing a ski mask and wielding a baseball bat, tethered a bomb-like device around her neck. It took bomb squad officers 10 hours to remove it. The device contained no explosives and Pulver was not injured.

The man left behind a note demanding money, along with an email address. New South Wales state police have said surveillance footage showed Peters in several locations where they believe he accessed the email account.

Peters, who traveled frequently between the United States and Australia on business, was arrested at his former wife's home in Louisville, Kentucky, about two weeks after the crime. He was extradited in September to Australia, where he has remained in custody.

The legal ordeal is over for fake collar bomb victim Madeleine Pulver after her attacker, Paul Peters, pleaded guilty.

Peters appeared in court by video from prison Thursday. He showed no reaction when his lawyer entered the guilty plea.

Why Peters targeted Pulver is unclear. U.S. federal court documents show Peters once worked for a company with links to her family, but the Pulvers have repeatedly said they don't know him.

At his U.S. extradition hearing in August, court documents from Australian police said a note attached to the chain attached to Pulver read:"Powerful new technology plastic explosives are located inside the small black combination case delivered to you. The case is booby trapped. It can ONLY be opened safely, if you follow the instructions and comply with its terms and conditions."

A man has been arrested in Kentucky for allegedly strapping a fake bomb around the neck of an 18-year-old woman in Australia that held her captive for 10 hours. NBC'S Sara James reports.

After X-raying the box several times and conducting other tests, bomb technicians determined it was harmless and removed it.

'Wrong place at the wrong time'Pulver, who has graduated from high school since the attack, and her parents were in court to hear the plea.

Her father, Bill, thanked police, prosecutors and members of the public for their support, and said the attack remains as mysterious and as "random to us in our minds as it did back on Aug. 3."

"We are incredibly pleased with today's outcome," Bill Pulver told reporters after the hearing. "It is great comfort knowing Maddie won't have to endure the stress and anxiety of reliving the events of that terrible night.

"Today's guilty plea brings closure to a crime that remains a mystery and as random to us in our mind as it did back on August 3."

New details have emerged about the man arrested in Kentucky for allegedly strapping a fake bomb to a teenager's neck in Australia and how police tracked him down. NBC's Sara James reports.

Pulver said his daughter was "in the wrong place at the wrong time."

"A poor decision by one man has prompted a truly extraordinary and inspiring response from many thousands of people and we will be forever grateful," he added.

A young woman in Sydney says a man wearing a ski mask strapped an explosive device to her neck. TODAY.com's Dara Brown reports.

Bill Pulver was once the president and CEO of NetRankings, a pioneer in tracking online exposure and readership for companies advertising on the Internet. He left after the firm was sold to ratings giant Nielsen in 2007. He is now CEO of Appen Butler Hill, a company that provides language and voice-recognition software and services.

Peters will appear in court next on March 16 for a pre-sentencing hearing. He faces a potential maximum sentence of 20 years in prison.

NBC News' Pete Williams, Reuters and The Associated Press contributed to this report.